It has been known that illuminating light is incident on a human's retina, and then, acts on a region called suprachiasmatic nucleus through a retinohypothalamic tract to thereby exert not only visual influence but also physiological influence. It has also been known that this effect is especially high in light having a lot of short-wavelength lights near 460 nm. For example, it is pointed that exposure to illuminating light at night inhibits secretion of hormone that is melatonin, which may affect a sleep quality. Therefore, a reduction of such influence has been demanded for the illumination used at night.
In order to reduce the influence, it is considered that the illuminating light does not contain light near 460 nm. However, this undesirably deteriorates color rendering properties that are significant functions for the illumination. The color rendering properties are an index indicating how close a color appearance of an object under a certain illumination is to a color appearance of the object under a standard light source. The measurement of the color rendering properties is specified under Japanese Industrial Standards JIS Z8726.
Evaluation methods of the color rendering properties include a general color rendering index and a special color rendering index. The general color rendering index is obtained by calculating color appearances of 8 colors, each having middle level of brightness and vividness, and having a whole hue. The special color rendering index is obtained by calculating color appearances of 15 colors, which include, in addition to 8 colors used for the general color rendering index, colors that reproduce a color and skin color more vivid than those of 8 colors. These values can be calculated by a value of a spectral distribution of the illuminating light.
There has been known a technique of reducing a non-visible influence of illumination with color rendering properties being kept beyond a certain level by solving an optimization problem of minimizing a value of melatonin secretory inhibiting amount under a restraint condition such that the value of the color rendering index is kept beyond a certain level.
However, in the related art, even when an object that does not reflect light near 460 nm is illuminated, for example, light near 460 nm has to be contained in a certain level in order to keep the value of the color rendering index beyond a certain level. Specifically, light with a wavelength unnecessary for keeping a color appearance of an object is contained, resulting in that the non-visual influence cannot efficiently be reduced.